As many have explained, you have a multiple entry non-o visa that is valid for one year from the date it was issued in November last year by the Thai Embassy in London.
If you actually look at the last entry stamp in your passport then you WILL see that this will have a date in it when you must LEAVE Thailand by, which is 89 days after you entered last.
You must now leave the country and then re-enter again in order to get a further 90 days, you cannot make a 90 day report.
Ignoring the advice of the many posters will mean that you will go on to overstay, ignore this for the next 90 days and you will be banned from entering Thailand for 12 months.
Take a look at that last entry stamp and see when you are permitted to enter until, I suspect that is today!